Whats with the change in my dog

I have had my 4 year old pitbull Becky for 3 years now. She has never shown a vicious side of her. The most she has ever done is bark like crazy when someone is outside of the house. My upstairs neighbor has a male dog Chase who Becky has been around for about a year now. Sometimes we dog sit and they have always gotten along beautifully. About 2-3 months ago I got another dog, Bear. He's a male Pomeranian. Becky gets along great with him. But since we got him Becky has gone after Chase 2 or 3 times viciously. Today she saw Chase outside and as I opened the door to enter my house she jetted outside and attacked Chase. She knows she did wrong because even hours later we have a hard time getting her to look at us and she's still (as I call it) cowering down as she walks. This time her face got so scratched up it's a little puffy. Does anyone have any idea what could be going on with her? Does it have anything do to with Bear?

she has taken it upon herself to be a guardian to the smaller dog. Sometimes as they get older they get possessive which seems to be the case. If they met off the property without the little dog she'd probably be fine.

he is your friend and protector, he will love you unconditionally, you owe it to him to be worthy of such devotion

Thanks for the quick replies. I figured it has to have something to do with Bear, being that that's when it all started but does Becky protecting him not have to make sense? We could be dog sitting, all 3 of them are in my house and they could very well be fine, like this past Sunday. Then yesterday Becky and Bear were in my house. Chase was outside, not a threat at all and as soon as I opened the door Becky was outside attacking Chase. I'm just trying to get the full understanding of this. Maybe because I'm not a dog I can't lol.

if Becky is unpredictable its best to keep chance away from her entirely. We don't know what triggers an attack and if sometimes she is ok and not other times she is unpredictable. And with an unpredictable dog you have to always be prepared for what she will do. So best to keep them away from each other unless its off both properties.

he is your friend and protector, he will love you unconditionally, you owe it to him to be worthy of such devotion

The Dog Whisperer, Cesar Milan would suggest walking all three together on several long walks so they can get to know and trust each other. Maybe your neighbor can join you, it would benefit her dog too. Maybe Chase on the outside, Bear in the middle and Becky on the other side. Then Becky can see that Chase is not a threat. Whoever walks Becky has to make sure she stays where she's supposed to be and doesn't lunge across for any type of attack. She must be stopped even before it happens, you can see the body language, nip that in the bud.

This exercise must be done in a calm and positive way, all dogs should feel calm and secure, walkers must feel confident and knowing.